Observations that carotenoid pigments could prevent lethal photosensitivity in bacteria, plants and animals suggested that the administration of carotenoids might ameliorate photosensitivity in man. A total of 75 patients with erythropoietic protoporphyria have been treated with oral beta-carotene. Over 80 percent of the treated patients have reported significant amelioration of their photosensitivity, and phototesting has confirmed objectively the patients' subjective assessment of improvement. Confirmations have appeared from several countries. It is now proposed to extend these studies in three general directions. First, the treatment of EPP will continue to be explored. Long-term effects of continued administration of beta-carotene will be studied by careful following of the treated patients. Alternative carotenoid treatment that might offer higher blood and skin levels or more effective photoprotection will be sought. Secondly, preliminary data in a few patients suggest that beta-carotene may be of benefit in forms of human photosensitivity other than EPP. The types of photosensitivity in which beta-carotene may be effective, and the possibility that other carotenoids may provide a broader spectrum of effectiveness, will be explored. Finally the basic mechanisms by which photosensitivity leads to cell damage in bacteria and other cell systems will continue to be examined on the cellular and subcellular level in photosensitized bacteria and animals, and further insight sought concerning the mechanism by which carotenoid pigments exert their protective function.